I2C driver : add a function to retrieve last transaction result

When a transaction is performed (in master **and** slave mode), internal state is set to I2C_STATE_READY once the transaction completes.

However, we'll never know if that transaction went fine, maybe it has stopped because the device could not identify any slave, so there could be many reasons why a transaction would stop/complete.
For instance, (non-exhaustive list), those reasons may be implemented :
* I2C_TRANSACTION_COMPLETED_FINE (everything went fine)
* I2C_TRANSACTION_COMPLETED_NO_SLAVE_FOUND (target slave did not respond, master mode only)
* I2C_TRANSACTION_COMPLETED_MAX_RETRIES_HIT (too much nack sent/received)